The main lines of almost all U.S. railroads employ welded track which is continuous except at insulated rail joints for electrical signals. Most insulated rail joints are comprised of fishplates or joint bars which are adhesively bonded to the rails in order to fix the gap and virtually to eliminate relative movement between the abutting rails. Typically the adhesives are two-part resin compositions which cure to a thermoset state.
Two-part systems are messy and difficult to use. The two-parts must be thoroughly mixed and then troweled onto a reinforcing fabric which provides sufficient bulk to insure adequate electrical insulation if the joint bars are metal. After the rails and joint bars have been heated by a torch, the resin-impregnated fabric is laid between the rails and joint bars, and bolts are inserted and tightened. If either the temperature is too high or there is a delay in completing the assembly, the resin can begin to gel before the bolts are tightened, thus resulting in an inferior adhesive bond.
Because of such problems, most railroads prefer to bond two sections of rail in a shop under well-controlled conditions, and subsequently to weld the bonded sections into the track. Not only is this a rather cumbersome, expensive procedure, but field welds tend not to be as reliable as the shop welds in the balance of the track.